LONDON (Reuters) -Chevron Corp said on Monday it would buy smaller rival Hess Corp in a $53 billion all-stock deal, taking its rivalry with Exxon Mobil to the next level by boosting its presence in U.S. shale and oil-rich Guyana. Given that Exxon Mobil had effectively taken itself out of the running for large-scale M&A following the recent Pioneer deal, we would have thought Chevron could bide its time, and hence we are a little surprised at the timing of this deal. It does address the concerns with Chevron's portfolio - namely over concentration in the Permian and lack of depth in growth elsewhere, however it comes at a price given the relative multiples between the two companies.